Nivolumab was first authorized at a weight-based dose (WBD) of 3 mg/kg every two weeks (Q2W). Since 2017, a fixed dose (FD) regimen [first 240 mg Q2W and then 480 mg per month (Q4W)] was allowed. The objective of the study was to compare a WBD regimen and an FD regimen with regard to effectiveness and safety. We conducted a single-center, retrospective, real-life study of consecutive adult patients who had received a WBD of nivolumab or an FD of 480 mg Q4W. The primary endpoint was the occurrence of grade ≥3 immune-related adverse events (irAEs). The secondary endpoints were overall survival and cost of the treatment. In all, 342 patients were included: 71 in the WBD cohort and 271 in the FD cohort. Of these patients, 201 patients (59.6%) experienced an irAE, and 24 of these events were graded as ≥3. At 12 months, there was no significant difference in irAE occurrence between the two cohorts [hazard ratio (95% confidence interval): 0.54 (0.21–1.36), P = 0.19]. The 12-month overall survival rate was significantly lower in the WBD cohort (P < 0.001). Switching from a fortnightly weight dose to a fixed monthly dose halves the cost of hospitalization. Our results did not show a significant difference between WBD and FD cohort in the occurrence of severe irAEs. However overall survival appeared to be significantly higher in FD group. Some clinical trials are investigating a hybrid dosing regimen in which a WBD is capped by an FD. The present results need to be confirmed in prospective studies.